The Regexp::Visualize::Simple generates a series of Graph::Easy
objects, which as a whole represents a regular expression parsed by
Regexp::Parser. Generated graphs can be exported to arbitrary
image format, such as PNG and SVG, supported by Graph::Easy. As an
input any regular expression dialect supported by Regexp::Parser or
its subclasses can be used.

It is sometimes impossible or undesired to represents a regular
expression as one complete graph. For example, if a regular
expression containing (?!...) segment were represented as a graph,
it would be difficult to understand. Therefore, the result
visualization might sometimes be split into multiple graphs. This is
why this module defines iterator methods has_regexp_node and
next_graph.

Generates a graph from the first node in the queue o the regular
expression node hold by the visualizer object.

If there is no node to be graphized in the queue, then a list of two
undef values is returned.

Otherwise, a list of the Graph::Easy object generated from the
regular expression node and the index number of the graph is returned.
The first graph generated by the visualizer object has the index of
zero (0), and the next graph has one (1).